The present invention relates to medical systems, and more particularly to a highly efficient integrated RF telemetry transmitter for efficiently communicating with and powering an implanted stimulator device, e.g., an implanted cochlear stimulator (ICS).
Implanted cochlear stimulator systems are known in the art. See, e.g., U.S. Pat. Nos. 4,267,410; 4,428,377; 4,532,930; 4,592,359; 4,947,844; and 5,569,307.
Any implantable medical device, including an ICS, requires operating power in order to perform its intended function. Most ICS devices receive operating power from an external control unit by way of an inductive or RF signal link. That is, the implanted unit includes a receiving coil. The external unit includes a transmission coil. During use of the system, the external transmission coil is positioned adjacent the implanted receiving coil and a high frequency carrier signal is applied to the external transmission coil. This carrier signal is coupled to the implanted receiving coil, even though there is no direct connection between the external and implanted coils, in much the same manner as an ac signal applied to a primary winding of a transformer is coupled to a secondary winding of the transformer, which secondary winding has no direct connection with the primary winding. Once received by the receiving coil, implanted circuitry rectifies the signal and converts it to a dc signal which is used as the operating power for the implant device. Moreover, modulation applied to the carrier signal provides a means for sending control signals to the implant device from the external device.
In recent years, a need and desire has arisen for the external unit of a medical system that controls and powers an implantable device, such as an ICS, to be smaller and lighter. In general, this means that the battery carried by the external unit, which powers both the external unit and the implant device, must also be smaller and lighter. In order to avoid the too frequent replacement and/or recharging of the battery carried by the external unit, and associated expense, this means that the operation of the circuits in both the external unit and implant device, as well as the transfer of power from the external unit to the implant device, must be made more efficient.
One of the circuit areas within the external unit that has heretofore proven to be relatively inefficient is the high frequency transmitter modulator. The high frequency transmitter modulator is the circuitry that allows a high frequency signal, e.g., the carrier signal, to be modulated with data prior to transmitting such signal to the receiving coil of the implant device. The design of such high frequency transmitter modulators has typically been founded on a sinusoidal design approach, used to make the actual transfer of signal energy from the external (transmitting) coil to the implant (receiving) coil more efficient. Disadvantageously, however, power amplifiers and other circuitry operating in a sinusoidal mode are notoriously inefficient from a power dissipation view point, and have traditionally required more components, and hence a larger size, than have other, e.g., digital and/or non-linear (non-sinusoidal) design approaches.
When the external unit has been realized in a large wearable unit that is carried on a belt or in a pocket of the user, connected by a long cable to a headpiece where the transmitting coil is housed, such inefficiency and larger size has been of little consequence. However, in order for a smaller external unit to be used, e.g., one that is worn behind the ear, such sinusoidal design approach, while theoretically effective at providing the desired function, results in less compact circuits that operate less efficiently than required. Thus, it is evident that there is a need for a high frequency transmitter modulator circuit that is both compact and efficient.
Coupled with the need for an efficient and compact high frequency transmitter modulator circuit is the need for an efficient and compact RF amplifier or driver circuit that receives the output signal from the transmitter modulator circuit and applies it to an antenna or transmitting coil for coupling to the implant device. Where the implant device also has the capability of sending telemetry signals back to the external unit ("back telemetry signals"), there is a similar need for a compact receiver circuit within the external unit to receive and process such back telemetry signals in an efficient, power-saving, manner.